Asteroids and
Meteorites
AST 111 Lecture 8?
Spherical chondrule made
of glass and feldspaths.
High interference
color olivine crystals.
Microscope images from JM Derochette
Hudson and Ostro
used radar images
obtained with the
Deep Space
Network Goldstone
radar antenna in
California and the
Arecibo telescope in
Puerto Rico in 1992,
when Toutatis
passed to within a
little more than 2
million miles of the
Earth.
Spatial distribution
The Asteroid Belt, Orbits
Inclinations
Hungaria
Phocaea
Eos
Koronis
Trojans
Distribution of Orbital Elements
• Most asteroids are located in the “main belt”
between 2.1 and 3.3 AU.
• The spread in eccentricities has an exponential tail,
suggesting some kind of equilibrium.
• Mean eccentricity is 0.14.
• Mean inclination is 15o.
• Many asteroids are on orbits that will not last more
than a billion years.
• NEO (Near Earth Object) orbits are unstable on
timescale of 107 years.
End states
• NEO’s will wind up in the Sun or ejected from the Solar
system, with smaller percentages hitting other objects (like
Earth or other planets).
• The end states of NEOs are governed by resonances with
high eccentricity regions which may be chaotic (3:1 and ν6).
• Almost all short period comets are eventually ejected from
the solar system. Some small percentage can be trapped in
the inner solar system.
• There are some objects in the asteroid belt which have
shown cometary behavior. For example Chiron has a
Saturn crossing orbit and has exhibited extreme variations
in brightness.
• Asteroids on highly eccentric or inclined orbits might be
dead comet remains.
Impact of impacts
Size *
Examples
Most recent
Planetary effects
Effects on life
Super-colossal
R > 2000 km
Moon-forming event
Mars
4.45×109 yr ago Melts planet
Drives off volatiles
Wipes out life on planet
Colossal
R > 700 km
Pluto, largest few
KBOs
t 4.3×109 yr ago Melts crust
Wipes out life on planet
Jumbo
R > 200 km
4 Vesta, 3 other
asteroids
~ 4.0×109 yr
ago
Vaporizes oceans
Life may survive below surface
Extra large
R > 70 km
8 Flora; 90 other
asteroids
~ 3.8×109 yr
ago
Vaporizes upper 100 m of oceans
Pressure-cooks troposphere
May wipe out photosynthesis
Large
R > 30 km
Comet Hale-Bopp,
464 asteroids
~ 2×109 yr ago
Heats atmosphere and surface to ~1000
K
Continents cauterized
Medium
R > 10 km
KT impactor
433 Eros (large NEA)
1211 other asteroids
6.5×107 yr ago
Fires, dust, darkness; atmospheric/
oceanic chemical changes, large
temperature swings
Half of species extinct
Small
R > 1 km
About 650 NEAs
~ 300,000 yr
ago
Global dusty atmosphere for months
Photosynthesis interrupted; few
species die; civilization
threatened
Peewee
R > 100 m
Tunguska event
103 yr ago
Major local effects
Minor hemispheric dust
Newspaper headlines
Romantic sunsets
From de Pater & Lissauer 2010, Lissauer 1999, and Zahnle & Sleep 1997.
Asteroid taxonomy
The composition of the surface of an
asteroid can be determined by
reflectance spectroscopy at ultraviolet,
visible and infrared wavelengths.
Broad classes (Bus & Binzel 2002):
❑ C group – carbonaceous, low
albedo (< 0.1).
❑ S group – silicaceous (stony),
3 Juno
1 Ceres
moderate albedo (0.1-0.25).
❑ X group – metallic, usually
4 Vesta
2 Pallas
moderate to large albedo.
The first four asteroids discovered, shown
And several “assorted” groups.
on the same scale as Earth and Moon
(NASA). Together they comprise 2/3 of
the mass of the asteroid belt.
C-group asteroids
C-type asteroids are the largest population: at least 40% of all
asteroids. They lie toward the outer part of the main belt.
❑ Dark, with albedo ~ 0.05; flat spectrum at red visible wavelengths.
❑ Reflectance spectra generally similar to carbonaceous chondrite meteorites (see below).
❑ A few show additional absorption at UV wavelengths and are given by some the classification G-type.
D-type asteroids currently appear to comprise about 5% of the total.
❑ Like Cs they are concentrated in the 1 Ceres, a C- (or G-) type
outer main belt but are seen further asteroid (HST/STScI/
out, too; e.g. among Jupiter’s Trojan NASA), the largest and
asteroids.
third brightest of the
asteroids.
C-group asteroids (continued)
❑ Ds are very dark – on average even darker than Cs – and
red, with featureless spectra: hard to identify composition.
❑ Distant + dark = hard to detect small ones. Thus we may
currently underestimate the size of this population.
❑ Recently a meteoritic analog of the Ds was found, with the
result that they appear even more primitive than Cs.
B-type asteroids are much rarer, until recently counting only 2
Pallas as a member (then called “U-type”).
❑ Though carbonaceous, Bs have higher
albedo and bluer color than Cs and Ds.
624 Hektor, perhaps the best known D-type asteroid (HST
image by Storrs et al. 2005)
S-group asteroids
S-type (stony) asteroids are the second most
numerous type: about 30% of all asteroids.
❑ Concentrated toward inner part of main belt,
with large albedos (~ 0.20); thus we may be
overestimating their fraction of the total.
❑ Reflection bands in the infrared similar to
those from pyroxenes and olivines.
❑ They are either thermally processed and
crystallized (like igneous rocks) or have been
“space weathered” by impacts and UV.
Adaptive-optical images and artist’s conception of 3 Juno, the
second-largest S-type asteroid (Harvard-Smithsonian Center for
Astrophysics)
S-group asteroids (continued)
Other S-group asteroids are rare, but some are still notable. They
differ from S-type by having much stronger mineral absorption
features near 1 µm wavelength.
❑ A-type: olivine
❑ Q-type: pyroxene and olivine
❑ R-type: pyroxene, olivine and plagioclase
❑ V-type: pyroxene; relative mineral abundances closely resemble
those of basaltic lavas (!). Until fairly recently the only member of the V type was its
eponym, 4 Vesta, which was more conventionally accounted
under the “U-type” (unclassifiable, or unique).
• Now there are a few more but all are tiny, and all are
members of Vesta’s orbital family (Vesta fragments?).
X-group asteroids
M-type (metal) asteroids comprise about 10% of asteroids.
❑ They’re shiny and relatively blue, with albedo ~ 0.20, but
lacking in silicate spectral features, so they’re probably rich
in metallic elements.
❑ Live mostly in the center of the main belt.
Artificially-sharpened Arecibo
radar images of 216
Kleopatra, not the largest Mtype but probably the most
famous (Steve Ostro, JPL).
X-group asteroids (continued)
P-type asteroids comprise about 5% of the total.
❑ Dark (albedos in the C-type range) and concentrated in the
outer main belt, but otherwise similar to M-type.
E-type asteroids are rare but prominent, as the observational biases are all in their favor.
❑ Highest albedos among asteroids (0.2-0.5) but otherwise spectrally similar to Ms.
❑ Concentrated on the inner rim of the main belt.
2867 Steins, an E-type asteroid (Rosetta/ESA).
Asteroid interiors
Not much is known for sure about asteroid interiors.
❑ A few of them are probably differentiated…
• Several S-group asteroids have bulk densities which exceed
the densities of the minerals which dominate their surfaces.
• One is 4 Vesta, which has a basaltic-lava surface.
❑ …but the only spherical one, 1 Ceres, doesn’t seem to be
differentiated.
❑ Many are so low in density that they must be quite porous, or
not really be very solid (rubble piles).
• This is consistent with the appearance of the craters in
planetary-probe flyby pictures of small asteroids: they tend
to look soft-edged, as if made in sand.
Typical small asteroids
Clockwise from right: 951
Gaspra (by Galileo), 253
Mathilde (by NEAR), and
25143 Itokawa (by Hayabusa)
(JPL/NASA and JAXA).
Asteroid Taxonomy
• Largest class (40%) are called C-type for carbonaceous. Very dark,
albedos ~0.04, flat spectrum in the red. UV absorption bands (hydrated
silicates). Similar in reflectance spectra to carbonaceous chondrites.
• S-type (stony) (30%). Brighter with albedo ~ 0.15. Absorption bands in
the infrared similar to those you might expect from iron and magnesium
bearing silicates (pyroxene and olivine). They are either igneous
(crystallized) or have been space weathered.
• D and P-types (5-10%) dark with spectra that are relatively featureless.
Reddish. Could be very primitive bodies with no proposed meteoritic
analog.
• M-type (M=metal). Albedo ~ 0.15, lacking silicate spectral features so
are probably irons. Include a W subclass with hydrate IR featuers.
• U-type. Weird and unclassified.
• E-type. Enstatite asteroids, near inner edge of belt. Very high albedo.
Asteroid type distribution
• No class of asteroids resembles ordinary chondrites. Is it
space weathering that has changed their expected spectrum?
Or do they tend to be small? Could be that S-type asteroids
include ordinary chondrites as well as stony asteroids.
• Taxonomy is correlated with spatial distribution suggesting
that some of the differences in asteroids are primordial. Ctypes tend to be in the outer asteroid belt. S-type tend to be
in the inner belt. D,P only in the extreme outer part and as
Trojans. M-type in the central regions.
Example asteroids
• Phobos and Deimos. These
are Mars’s moons, however,
their spectral properties are very
similar to C-type asteroids.
• They are small enough that they
are not round.
• Phobos is heavily cratered and
has cracks, possibly caused by a
large impact.
• Deimos is less cratered, possibly
because it is a rubble pile with a
lot of sediment.
Strength
Pressure at core Pc~GM2/R4
If core pressure exceeds material strength then the
body deforms until it is spherical
Material strength of ice ~ 3GPa
Material strength of rock ~ 50GPa
Gaspra: Flora family. S-type, heavily
cratered. Grooves and albedo
variations. Regolith?
Ida and Dactyl: Koronis family. Ida is an
S-type asteroid with a very small moon
(Dactyl). Spectral differences, Dactyl
has more pyroxene. Degraded craters,
suggesting old age. Dactyl would be
disrupted in 108 years from an impact.
It could be reformed during collisions.
Mathilde: C-type. Very low density
suggesting that it is porous. Smooth
albedo, suggesting that it is
homogeneous and undifferentiated. No
evidence for hydrates.
Special features: asteroids with moons
131 asteroids have been observed to have
satellites, since the first one in 1993 (Ida/
Dactyl, by Galileo).
❑ Five of them have two moons.
❑ 90 Antiope is a double asteroid, with
nearly equal-mass components.
❑ The moons have provided a means by
which to measure masses of asteroids,
and thus to provide much more
accurate average densities. (Not as
accurate as we’d like, because the
asteroids tend to be so irregular that it’s
hard to estimate their volume.)
Infrared AO image of 90
Antiope (Bill Merline et
al., SWRI), whence was
determined the
separation of 160 km
and density 0.6 gm cm-3
(!).
Special features: dynamical families and fragmentation
In 1918, the first great Japanese astronomer, Kiyotsugu Hirayama,
discovered three “families” of asteroids, the members of which
have very similar orbits, much too similar for the agreement to be
a result of random chance. Koronis, Eos and Themis are his three
original groups.
❑ Same reasoning that Olbers tried to apply to all asteroids.
❑ Hirayama concluded that each of these groups consists of
fragments of a larger asteroid that broke apart, that
subsequently were entrained into their similar orbits by the
influence of Jupiter.
❑ With typical asteroid sizes and speeds, most collisions should
be explosive, so we expect such groups.
❑ Family members turn out all to have very similar spectra and
composition.
Meteors and meteorites
A meteorite is a rock that has fallen from the sky.
❑ It’s called a meteor while it’s falling through the sky. If a rock
associated with a meteor is found it is called a fall; otherwise,
it’s called a find.
❑ Only after falls were well observed and documented by
Chladni (1794) and Biot (1803) did it become accepted that
they did actually fall from the sky. Before that, the idea was
considered by the educated to be as crazy as sightings of
spacecraft piloted by extraterrestrials are today.
• And the latter is demonstrated to be crazy, of course.
❑ Meteorites turn out to come mostly from asteroids and are
relatively un-thermally-processed, so they preserve a record
of the state of solid matter in the early solar system.
Meteors from the 2004 Perseid shower, by Fred Bruenjes.
Meteorite classification
Irons
❑ Guess what those are made of.
❑ Meteorites without a lot of iron are
known as stones. There are also stonyirons...
• …like pallasites, which consist of
gem-quality olivine crystals
embedded in a nickel-iron matrix. Section of an iron meteorite,
❑ Irons contain nickel and iron along with polished and lightly acidsiderophile elements (those that alloy etched, in the New England
Meteoritical Services
well with iron, like gold and silver.)
collection. Note the
❑ Irons and stony-irons come from
distinctive Thomsondifferentiated parent bodies.
Widmanstätten patterns.
Meteorite classification (continued)
Part of the Krasnoyarsk meteorite, the first
pallasite found (Pallas, 1776). From
MeteoriteCollector.org.
Thin, polished section
of the Esquel pallasite
(Wikimedia
Commons).
Meteorite classification (continued)
Achondrites
❑ These are rocky, nonmetallic pieces
resembling the earth’s crust, and
lacking chondrules (see below).
❑ Mostly composed of silicates and
iron-nickel oxides.
❑ Enriched in lithophile (easily
incorporated in silicates) or
chalcophile (alloying well with
copper) elements.
❑ Significantly depleted in iron and
siderophile elements.
❑ Must also come from differentiated
parent bodies.
Microscopic image of a thin section
of a eucrite achondrite containing
mostly plagioclase and (more
birefringent) pyroxenes. By J.M.
Derochette.
Meteorite classification (continued)
Chondrites
❑ Structurally most primitive of meteorites.
❑ They are called chondrites because they contain chondrules, (for which, see below).
❑ Have never completely melted, though they have been modified in some cases by aqueous and/or thermal processes, and have igneous silicate and metal inclusions in close proximity.
❑ Have abundances of elements that belong
to nonvolatile molecules which are
precisely the same as those of the Sun, in stark contrast to irons and achondrites.
• Thus chondrites come from non-
differentiated parent bodies.
Two fragments of the
Allende meteorite.
Photo by Brian
Mason, Smithsonian
National Museum of
Natural History.
Classes of chondrites
❑ Volatile-rich ones, containing several percent of carbon, are
called carbonaceous chondrites. Of this type there are
slight differences in composition which leads to subtypes
such as CI, CM, CO and CV.
• Allende is a CV carbonaceous chondrite.
• The extra letter refers to the meteorite that was the first
or best example; e.g. Murchison for the CMs.
❑ Ordinary chondrites: classified based on their Fe/Si ratio
(H =high Fe, L=low Fe, LL = low Fe, low metal, mostly
oxidized metal).
❑ Enstatite chondrites are dominated by that mineral.
Classified based on iron abundance into subclasses EL and
EH.
Carbonaceous chondrites
CAI
Chondrules
Depleted in meteorite
Depleted in the Sun
Matrix
The Allende meteorite. Left: cross section (NASA/JSC). Right: Element
abundances, compared to those in the Sun (de Pater & Lissauer 2010).
Ordinary chondrites
Chondrule
Chondrite H5 Sahara 97095, by J.M. Derochette.
Falls and finds, by type
Type
Ordinary chondrites
Carbonaceous chondrites
Other chondrites
Asteroidal achondrites
Martian meteorites
Lunar meteorites
Stony-irons
Irons
Falls
Finds, N
Finds
N
%
Antarctica Elsewhere
%
853 79.9
26655
9687
87.4
43
4.0
921
464
3.3
18
1.7
407
214
1.5
89
8.3
776
888
4.0
4
0.4
25
74
0.2
0
0.0
33
113
0.4
11
1.0
82
177
0.6
49
4.6
144
911
2.5
Totals as of 6 October 2011, from the Meteoritical Bulletin Database: 42,638
validated meteorites, of which 87.2% are ordinary chondrites.
❑ The vast majority of these reside in museums and research labs; private
collectors account for a large additional total.
Chondrules
Chondrules are 0.1-2 mm diameter,
spherical, often glassy igneous
inclusions which required high
temperatures (T = 1500-1900 K) to
form.
❑ Because some are glassy they
probably melted and cooled
very fast: minutes to hours.
❑ There is a correlation between
chondrule size and
composition, suggesting that
they were not well mixed before
incorporation into larger
bodies.
Olivine chondrule, mostly
surrounded by carbonaceous
matrix; again by J.M. Derochette.
CAIs
Chondrites also have calciumaluminum-rich inclusions
(CAIs) which form at higher
temperatures than chondrules.
❑ Ca-Al rich anorthite, melilite,
perovskite and forsterite,
mostly.
❑ Thought to be the oldest
solids in the solar system:
~1.7 Myr older than
chondrules in the same
chondrite (Amelin et al. 2002,
Connelly et al. 2008).
(Sasha Krot, U. Hawaii)
Meteorite recovery
Lots of meteorites are found, well preserved and concentrated,
in Antarctica. Some deserts provide good samples too.
❑ Suppose you were walking around in the plains of
Antarctica, and came upon a rock laying on the surface.
What were its options for getting there?
❑ Same holds for desert plains, like deep in the Sahara. If running water couldn’t have brought the rock there, it might be a meteorite.
Source regions: large bodies
Whence come the meteorites?
❑ Some meteorites are exactly the same as lunar rocks
(anorthosite breccias); they must be from the Moon.
❑ The SNC class includes three types that come from Mars:
• The most convincing evidence is the noble gas
abundances, which are distinctive and the same as those
measured by the Viking landers.
• One, ALH84001, became infamous: a 4.5 billion year old
Martian achondrite meteorite recovered from Antarctica,
with magnetite which has been interpreted as evidence for
life on Mars.
❑ Impacts on rocky Solar-system bodies can eject rocks which
can travel to Earth, particularly from Mars and the Moon
because of their lower surface gravity.
Source regions: smaller bodies
But 99.4% of meteorites are from bodies
smaller than the terrestrial planets.
❑ Reflectance spectra of classes of
meteorites match reflectance spectra of
classes of asteroids well.
❑ Comets and asteroids are the two major
classes of parent body populations for
chondrites.
• Of these the C-group asteroids
dominate by a wide margin, but the
dividing line is somewhat indistinct.
❑ Achondrites and irons clearly come from
the asteroid belt (Ss and Xs).
• 63% of achondrites – the “H-E-D”
classes – are from 4 Vesta alone (!).
Morrison & Owen 1996
Ages of meteorites
Because they commonly contain silicate minerals, meteorites can
be radioactively dated, just like rocks.
❑ Result: they all turn out to be very old – even older than moon
rocks – and similar in age.
❑ Example: the CAIs in the Allende meteorite (a CV3) are
4.5677±0.0009×109 years old (Connelly et al. 2008).
❑ This pretty much determines the age of the solar system.
• CAIs are oldest solids found; it is thought that the pre-solar
nebula itself formed only 104-105 years earlier.
❑ Moon rocks are younger (3-4.45×109), so have melted since
then. Terrestrial rocks are all less than 4×109 years old.
❑ Differences in composition tell us about where they formed
(mass fractionation), nuclear decay, processing by melting and
water and cosmic rays.
Ages of meteorites (continued)
Ages of chondrules and CAIs
in Allende, derived from U-Pb
radioisotope dating (Connelly
et al. 2008). U-Pb is the isotope
system currently favored for
use on the oldest meteorites,
as Rb-Sr is for the oldest
terrestrial and lunar rocks.
Note the significant difference
in the ages of chondrules and
CAIs.
Basic classification-Irons
• Most people are familiar with chunks of metal.
These meteorite are known as irons.
• Those without a lot of iron are known as stones.
There are also stony-irons (a mix).
• Irons contain nickel and iron along with
siderophile elements (those that like to combine
with iron like gold and silver.)
• Irons and stony-irons come from differentiated
bodies.
Achondrites
• Differentiated rocky non metallic pieces
resembling the earth’s crust are called achondrites.
• Silicate and oxide composition
• Enriched in lithophile or chalcophile elements
rather than siderophile elements.
• Significantly depleted in iron and siderophile
elements compared to Solar composition.
Achondrites Millbillillie Eucrite
On this thin section,
two main minerals
are present: low
relief and low
birefringence
plagioclase
and high relief and
higher birefringence
pyroxene.
Again microscopy
by JM Derochette.
Primitive meteorites
• Also known as chondrites.
• Have never melted.
• Modified in some cases by aqueous processes and by
thermal processes.
• Silicates and metals in close proximity.
• The are called chondrites because they tend to contain
chondrules.
• Composition provides the best estimates of average solar
system composition of elements.
• As far as we know, the abundances of non gas elements in
chondrites is exactly the same as that of the Sun.
Chondrites
L6 chondrite JALU (Libya)
Polished section of a chondrite H5 with an oblique fiber optics illumination. The opaque minerals
appear dark on this image.
Nice chondrule at the top left. Chondrite
H5 Sahara 97095.
Chondrules
• Chondrules are spherical glassy igneous
inclusions.
• Because they are glassy cooled quickly.
• There is a correlation between chondrule
size and the types of crystals in them
suggesting that the cooling rate is somehow
related to the size of the globule.
Classes of chondrites
• Volatile rich containing several percent of carbon
“carbonaceous chondrites”. Of this type there are
slight differences in composition CI, CM, CO and
CV types.
• Ordinary chondrites Classified based on their Fe/
Si ratio (H =high Fe, L=low Fe, LL = low Fe, low
metal, mostly oxidized metal).
• Enstatite chondrites which are dominated by that
mineral. Classified based on iron abundance EL
and EH.
Processing of chondrites
• They are primitive but could have suffered
metamorphisms, shocks, brecciation (breaking up
and reassembly), chemical processing such as by
water.
• Given a petrographic type: Range from volatile
rich type 1 to volatile poor type 6.
• Since type 1,2 are processed chemically by water,
they are more altered than type 3.
• Type 7 have undergone partial melting.
Brenham : PALLASITE: olivine
crystals cemented in a kamacite and
taenite matrix. Analogous terrestrial
rocks are not known, but are
possible. (JPEG, 458 K) (Ward's catalogue, Rochester, NY)
Iron meteorites are classified
according to their nickel and trace
elements of germanium and gallium.
Compositional differences are
correlated with crystalline properties.
Pallasites are igneous and formed at
the interface of molten metal and
rock where olivine can crystallize in
the metal.
Source regions-Large bodies
• Some meteorites are exactly the same as lunar rocks
(breccias).
• SNC class believed to be Martian.
• The most convincing evidence is the noble gas abundance
which is the same as that measured by the Viking landers.
• ALH84001 infamous 4.5 billion year old Martian meteorite
recovered from Antarctica, with magnetite which has been
interpreted as evidence for life on Mars.
• Impacts can eject rocks which can travel to Earth
particularly from Mars and the Moon because of their lower
surface gravity.
Source regions-smaller bodies
• 99% of meteorites are from smaller bodies.
• Comets and asteroids are the two major classes of expected
parent body populations.
• This dividing line is now more obscure because of the
different classes of comets and the Kuiper belt. And there
could be extinct comets.
• Achondrites and irons are clearly from the asteroid belt.
• Spectral classifications match reasonably well. In other
words, spectral information observed from asteroids are
consistent with what you would expect from meteor
composition.
Micrometeorites
• In the 1950s, Dr. Fred L. Whipple predicted that very small
particles from comets could survive entry into the Earth's
atmosphere. These fragments are less than 2/1000ths of an
inch across, and gradually descend through the atmosphere
to land on Earth. Such particles were first collected using
high altitude balloons, but because the particles are so rare
in the upper atmosphere, only 10 samples were collected.
• Beginning in the 1970s, Lockheed U-2 aircraft were used to
collect samples of the upper atmosphere. Plates coated with
silicone oil were mounted in airtight containers on the wing
tips, and were exposed to the air stream when the plane
reached a cruising altitude of 60,000 feet.
Dust
Photos courtesy of Donald
Brownlee, Univ. of Washington.
Because the particles are so small,
their surfaces can only be studied
in detail using electron
microscopy. This scanning
electron microscope photograph
shows a grain about 10 microns (.
0004 in) wide, which has a very
open structure, presumably once
filled by icy comet material. As
these particles are ejected from
comets, the ice immediately
sublimes (turns from solid to gas),
leaving a framework of dust
similar in comparison to ancient
carbon-rich meteorites.
Fall Phenomena
• For the Earth impact velocities range from
11 to 73 km/s.
• Interactions with upper atmosphere can slow
down small particles.
• Micrometeorites are able to reradiate the
heat. (smaller than 100 microns).
• They have been recovered both on the
ground and from jets.
Fall Phenomenon-larger objects
• The surface of the meteor is heated by an atmospheric shock front.
• It cannot radiate away the energy if it is above a particular size and
velocity.
• It gets hotter than 2000K at which point rock melts.
• Liquid evaporates off the surface. “Ablation” cools the object.
• Heat of ablation is Q~5 ×1010 erg g-1 for stones and irons.
2
2
dE
dm
1
3 " v − vc #
=Q
= − CH ρ atmos Av %
&
2
dt
dt
2
v
'
(
CH "heat transfer coefficient" read fudge factor ~0.1
ρ atmos density of air.
A surface area of object
v velocity of object.
vc ~ 3km/s, is the velocity below which ablation isn't important.
Atmospheric Entry
• The outer part of the meteor gets hot enough to
melt rock.
• However it falls so fast (and conductivity is fairly
slow) that the rock only melts to a depth of a few
mm (for stones) ~1cm for irons which have higher
conductivity.
• If the object survives ablation then when it hits the
ground it is cold.
• The crust is still altered (becoming magnetized for
irons) but the interior may be undisturbed.
Velocity of the meteorite
TheThe
meteor
pressureduedue
to drag
meteorfeels
feels aa pressure
to drag
of
P ⇠ CD ⇢atmos v 2 /2
P ~ CD ρ atmos v 2 / 2 where v is the velocity,
𝜌atmos atmospheric density
If the pressure causesρ atmos
a stress
exceedsdensity
is thethat
atmospheric
and
CD ~1 drag coefficient
strength of material Cthen
the meteor
D ~1 a drag coeficient
If the pressure
exceeds
the tensile
strength of the
fragments
resulting
in a strewn
field
𝑣 material
relative velocity
the meteor can fragment resulting in a strewn field.
Force
is pressure times area A
Force is P times area A
dv
2
dv
2 =
m
C
⇢
v
m
= −CD ρ atmos v A / 2 + mgD atmos A/2 + mg
dt
dt
How do we figure out when v changes significantly?
dv v
1 v
When
~ where t is the transit time. ~ .
dt t
t h
v2
m ~ CD ρ atmos v 2 A / 2 → m ~ ρ atmos Ah = mass of atmospheric column.
h
As a guideline, if the meteor encounters it mass in the atmosphere
If the
meteor sweeps up its mass in the atmosphere
it sweeps up, it will loose a substantial fraction of its incoming velocity.
then it looses a large fraction of its incoming velocity
Asteroid 2008 TC3
first asteroid predicted to impact Earth prior to
impact
Map of the Nubian Desert of northern Sudan with the
ground-projected approach path of the asteroid 2008 TC3
and the location of the recovered meteorites. Credit: P.
Jenniskens, et. al
Asteroid exploded in the air, leaving a trail of debris
Explosions/Fireballs
• Bolides (fireball, then bright explosion at the end of a streak)
• Famous Tunguska explosion 1908
• Mechanical and Thermal stresses (pressure difference on
different sides of body, pressure from heated volatiles)
• Brighter it is in the sky, the less likely pieces get to the
ground.
• Small cometary icy materials apparently common, more
likely to explode cause fireballs than leave falls
Review
❑Classification of asteroids
❑Classification of meteors
❑Chondrites, Achondrites and Irons.
❑ Micrometeorites
❑ Fall phenomena